Generally, a conventional battery charging system, e.g., a Li-Ion battery charging system, has three charge periods including a precharge period, a constant current (CC) charge period, and a constant voltage (CV) charge period. Referring to FIG. 1, a charging profile 100 of a conventional battery charging system during the three periods is illustrated.
As shown in FIG. 1, the charging profile 100 includes a charging current profile 102 of a battery cell and a voltage profile 104 of the battery cell. The charging current profile 102 changes with the voltage profile 104 during the three charge periods described above. During the precharge period, the battery cell is charged with a small precharging current as shown in the charging current profile 102, and the battery cell voltage can increase slowly as shown in the voltage profile 104. When the battery cell voltage reaches a voltage threshold of a CC mode marked in the voltage profile 104, the battery charging system is transferred to the CC charge period. During the CC charge period, the battery cell is charged with a constant current as shown in the charging current profile 102, and accordingly the battery cell voltage can increase rapidly as shown in the voltage profile 104. When the battery cell voltage increases to a voltage threshold of a CV mode marked in the voltage profile 104, the battery charging system is transferred to the CV charge period. During the CV charge period, the battery cell will be charged with a current decreasing gradually as shown in the charging current profile 102 to keep the voltage of the battery cell constant and equal to the voltage threshold of the CV mode.
Power dissipation of a charging switch in a battery charging system can be expressed as ICC(VIN−VBATT), where ICC represents the constant current and (VIN−VBATT) represents a voltage difference between a power source voltage, e.g., a voltage of an alternating current (AC) adaptor or a Universal Serial Bus (USB) port, and the battery cell voltage. In a battery charging system with a linear charger, a thermal issue may arise during the CC charge period since the value of ICC*(VIN−VBATT) may be relatively high when the battery cell voltage is relatively low. This thermal issue may trigger the battery charging system's thermal protection mechanism, causing the battery charging system to stop charging until the temperature cools down enough. Under some circumstances, the battery charging system may stop charging after a relatively short period of time. This in turn may cause the battery charging system to quickly oscillate between charging and not charging, which may decrease the efficiency of the battery charging system.